Question

the process in which the hotter parts of a gas or liquid rise to the top and the cooler, denser parts sink.

Explanation:

The description given is the definition of convection, a concept in the field of Physics (under Natural Science). Convection is the transfer of heat through the movement of fluids (liquids or gases) where warmer, less dense parts rise and cooler, denser parts sink, creating a circulation pattern.

Answer:

Convection